アオイガイ (Common Paper Nautilus)

<em>Argonauta argo</em> is a pelagic cephalopod mollusc in the family Argonautidae, found in open tropical and subtropical ocean waters across Asia and Europe, with records from Norway, Portugal, and Taiwan. Unlike true nautiluses, it belongs to the octopus lineage and produces a thin, papery egg case secreted by the female's webbed arms, which is commonly mistaken for a shell. The species inhabits surface to mid-water oceanic environments and is typically encountered near coastlines following storms or during seasonal current shifts. Females are significantly larger than males and carry the egg case containing developing embryos. The species is carnivorous, typically feeding on small crustaceans, zooplankton, and other small marine invertebrates. The IUCN classifies this species as Least Concern given its wide pelagic distribution. Its occurrence in Norway likely reflects occasional drift individuals carried northward by Atlantic currents rather than a breeding population. Biological traits such as average lifespan, body length, and body weight are not consistently recorded across populations in standardized databases, and detailed dietary studies remain poorly documented at the species level. <em>Argonauta argo</em> is often collected as a natural curiosity for its beautiful egg case.